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Working with promises inside an if/else

I have a conditional statement in which I need to perform one of two operations, then continue after whichever operation has resolved. So my code currently looks as follows:
if (shoud_do_thing_a) { //should_do_thing_a is just a variable that determines which function to call. it is not a promise
do_thing_a()
} else {
do_thing_b()
}
// more code
The issue is that both do_thing_a and do_thing_b return promises, and I can't move on until whichever gets executed has resolved. The best way I've come up with to solve this is like this:
var more_code = function () {
// more code
}
if (shoud_do_thing_a) {
do_thing_a().then(more_code)
} else {
do_thing_b().then(more_code)
}
I don't like this structure. It's difficult to follow because you need to jump around to find where more_code is defined (imagine I have this type of control flow in several locations), rather than simply being able to continue reading.
Is there a better way to deal with this type of thing in javascript?

If you can use async/await
async function someFunc() {
var more_code = function () {
// more code
}
if (shoud_do_thing_a) {
await do_thing_a()
} else {
await do_thing_b()
}
more_code()
}
Or if you can't, use then():
var more_code = function () {
// more code
}
var do_thing;
if (shoud_do_thing_a) {
do_thing = do_thing_a()
} else {
do_thing = do_thing_b()
}
do_thing.then(more_code)

If you're stuck with raw Promises and can't use async/await (You usually should have no trouble, what with babel/typescript etc), the following is a bit more elegant than storing the promise in a variable:
function something() {
return Promise.resolve()
.then(() => {
if (should_do_thing_a) {
return do_thing_a();
}
else if (should_do_thing_b) {
return do_thing_b();
}
})
.then(some_more_code);
}
Note that when you start working with Promises, your functions should always return a Promise that other functions can work with. Leaving an asynchronous action without any way to handle it means bad things, especially when it comes to error handling.
In a more general sense, it means that when you use Promises, more of your code is "uplifted" into being executed and returned as Promises.

How I want to improve on other answers:
keep it clean and simple
no unneeded variables
return promise asap
in js we use camelCase
put it in a function and name that function to keep it readable
let then execute moreCode so it's called after the thing is done.
function doTheThing () {
if (shouldDoA) return doThingA()
else return doThingB()
}
doTheThing().then(moreCode)

Simple working example:
The scope it's defined in must be async.
const createUser = async (data) => {
if (await isUsernameTaken(username)) { return 'username-taken' }
}
The isUsernameTaken func:
const isUsernameTaken = async (username) => {
const request = await API.SomeRequest
return !request.isEmpty
}

Save the promise and add the then after the if statement:
var promise;
if (shoud_do_thing_a) {
promise = do_thing_a();
} else {
promise = do_thing_b();
}
promise.then(more_code);

var promise = shoud_do_thing_a? do_thing_a: do_thing_b
promise().then(function () {
// more code
})

Similar to other answers here, but you can self execute the async and clean up the condition a bit.
(async () => {
const should_do_thing_a = true
const do_thing_a = function() {
return new Promise(function(resolve, reject) {
resolve('a')
})
}
const do_thing_b = function() {
return new Promise(function(resolve, reject) {
resolve('b')
})
}
const result = (should_do_thing_a) ? await do_thing_a() : await do_thing_b()
console.log(result)
})()

The way I would do it would be to put the if check into another function that returns a promise. The promise gets resolved with the resolve of the other function calls in the if-else statement.
Example:
function performCheck(condition) {
var defer = $q.defer();
if (condition) {
doThingA().then(function(response) {
defer.resolve(response);
});
} else {
doThingB().then(function(response) {
defer.resolve(response)
});
}
return defer.promise;
}
performCheck(condition).then(function(response) {
//Do more code.
});
In my opinion, I would prefer this method because this function can now be used in multiple places where you have a check on the condition, reducing code duplication, and it is easier to follow.
You could reduce this down further with
function performCheck(condition) {
var defer = $q.defer();
var doThisThing = condition ? doThingA : doThingB;
doThisThing().then(function (response) {
defer.resolve(response);
});
return defer.promise;
}
performCheck(condition).then(function(response) {
//Do more code.
});

You can use async/await
async function fn() {
let p, result;
if (shoud_do_thing_a) {
p = await do_thing_a()
} else {
p = await do_thing_b()
}
if (p) {
result = more_code();
}
return result
}

more_code = miFunc() => return new Promise((resolve, reject) => ... });
Solution 1
const waitFor = should_do_thing_a ? do_thing_a() : do_thing_b();
waitFor.then(...).catch(...)
Solution 2
let waitFor = Promise.resolve();
if (do_thing_a) {
waitFor = do_thing_a();
} else {
waitFor = do_thing_b();
}
waitFor.then(...).catch(...);

Access value that promise .then returns outside of promise

Title says it all, I am using the async-library eachSeries method
let
valueOne = null,
validItem = null;
const asyncEachSeries = require('async/eachSeries');
const getContent = function (item) {
contentResponse = fetchContent(item);
return contentResponse;
}
if (recommendationsArr.length > 0) {
asyncEachSeries(recommendationsArr, function (item, callback) {
getApiContent(item).then(function getValidResult(response) {
try {
valueOne = response.content.valueOne || null; //may or may not be present
recommendationsArr.splice(recommendationsArr.indexOf(item), 1);
validItem = item;
console.log("##### valueOne is: ", valueOne); // I get data here
return true;
} catch (err) {
valueOne = null;
return false;
}
});
//I am guessing I need a return statement here, though not sure what
});
console.log("##### Outside promise, valueOne is: ", valueOne); // no data here
// Access valueOne, validItem, recommendationsArr to be able to pass as props for the component
return {
component: <Layout><ComponentName
//pass other non-dependent props
validItem={validItem}
valueOne={valueOne}
recommendationsArr={recommendationsArr}
/></Layout>,
title: `Page Title`,
};
}
return null;
Scenario: recommendationsArr is an array of items (Its 99.9% not null, but since its an external api call I prefer to have a check). The purpose is to have values for valueOne, validItem, and an updated recommendationsArr.
The validity depends on the existence of valueOne, so if the recommendationsArr[0] has valid valueOne then I don't need to fetch api results for the rest of the array. I am using eachSeries as it runs only a single async operation at a time, hence if that async gets me the valid result I don't have to iterate over other items.
Sample recommendationsArr: recommendationsArr = ["blue", "white", "green", "red"]; //usually the array is anywhere between 12-50 elements
Now in order to be able to pass the updated values to the component I need to be able to access the values set in try block outside of the asyncEachSeries iteration loop. I understand I will have to return the processed/new values but am not sure at what point and how to return those values.

As is usual with any question concerning asynchronous code you return values by using the callback. But eachSeries does not collect return values. What you want is async.mapSeries:
async.mapSeries(recommendationsArr, function (item, callback) {
getApiContent(item).then(function (response) {
try {
valueOne = response.content.valueOne || null; //may or may not be present
recommendationsArr.splice(recommendationsArr.indexOf(item), 1);
validItem = item;
callback(null, valueOne); // This is how you return a value
// return true; // return does absolutely nothing
} catch (err) {
valueOne = null;
callback(err, valueOne);
// return false;
}
});
},function (err, result) {
// this is where you can process the result
// note that the result is an array
});
However, since you are using promises you can use async-q instead which wraps caolan's async library in promises:
var asyncQ = require('async-q');
asyncQ.mapSeries(recommendationsArr, function (item) {
// Note: the return in the line below is important
return getApiContent(item).then(function (response) {
try {
valueOne = response.content.valueOne || null; //may or may not be present
recommendationsArr.splice(recommendationsArr.indexOf(item), 1);
validItem = item;
return valueOne; // this is how you return a value in a promise
} catch (err) {
return null
}
});
})
.then(function(result) {
// you can process the result here
});
Personally I'd prefer to rewrite the code and process the results at the end because the code is cleaner:
asyncQ.mapSeries(recommendationsArr, getApiContent)
.then(function (result) {
for(var i=0;i<result.length;i++) {
var response = result[i];
// do what needs to be done with each response here
}
});

Q promises chaining to do things in correct order

I have a asynchronous function that needs to be called multiple times in the correct order. It's about uploading images to a server but like I said the images should be uploaded in the correct order.
My function looks like this:
function uploadImage(sku,fileName) {
console.log("Uploading image for sku="+sku+", imageName="+fileName);
var deferred = Q.defer();
var readStream = fs.createReadStream(rootPath+'/data/'+sku+"/"+fileName);
var req = request.post('http://localhost:3000/'+sku+'/upload/'+fileName);
readStream.pipe(req);
req.on('end', function() {
console.log("Image imageName="+fileName+" uploaded successfully.");
db.updateLastUploadedImage(sku,fileName).then(function (res) {
if(res) {
console.log("Table watches for sku="+sku+" updated.");
deferred.resolve(sku);
}
});
});
req.on('error',function(err) {
deferred.reject(err);
});
return deferred.promise;
}
I tried to bring it on with chaining the promises like documented in https://github.com/kriskowal/q but it's not working well. Somehow I do not come to the "then" block.
So I tried to make a recursive function but it also does not go inside the "then" block of the function call.
Only this method works but its not running through the correct order.
function uploadImages(sku) {
var promises = [];
for(var x=0; x<10; x++) {
promises.push(uploadImage(sku,(x+1)+".jpg")));
}
return Q.all(promises).then(function (res) {
return sku;
});
}
My recursive solution looks like this:
function uploadImages(sku,current,max) {
var deferred = Q.defer();
if(current<=max) {
uploadImage(sku,current+'.jpg').then(function (res) {
if(res) {
uploadImages(sku,current+1,max);
}
}, function (err) {
deferred.reject();
});
} else {
deferred.resolve(sku);
}
return deferred.promise;
}
What I'm looking for is something like this (but thats not the way to implement):
return uploadImage(sku,"1.jpg").then(function(res) {
return uploadImage(sku,"2.jpg").then(function(res) {
return uploadImage(sku,"3.jpg").then(function(res) {
return uploadImage(sku,"4.jpg").then(function(res) {
return uploadImage(sku,"5.jpg").then(function(res) {
return uploadImage(sku,"6.jpg").then(function(res) {
return uploadImage(sku,"7.jpg").then(function(res) {
return uploadImage(sku,"8.jpg").then(function(res) {
return uploadImage(sku,"9.jpg").then(function(res) {
return uploadImage(sku,"10.jpg").then(function(res) {
return sku;
});
});
});
});
});
});
});
});
});
});
So what is the best practice for my purpose?

There is no concept of "correct order" for async calls because they are just that -- asynchronous and they can end at any point.
In your the callback in Q.all(promises).then(...) you should have the responses in the order that you made them, but the order of your console logs may not be in the same order due their asynchronous nature.
In your case, you can probably do it recursively:
function uploadFiles(sku, current, max) {
return uploadImage(sku, current + '.jpg').then(function (sku) {
if (current > max) { return sku; }
return uploadFiles(sku, current + 1, max);
});
}
// use it
uploadFiles('SOME_SKU', 1, 10).then(function (sku) {
// ALL DONE!
});

Try to catch exceptions from the promise.
return Q.all(promises).then(function (res) {
return sku;
})
.catch(function (error) {
// Handle any error from all above steps
});

Nodejs Promise for custom api

I am new to nodejs and using promise and actually this is my first real app with nodejs.
So i have been reading all day and i am a bit confused.
So this is my module :
function User() {
var self = this;
self.users = {};
self.start = function (user, botId) {
return new Promise(function () {
return get(user).then(function (data) {
debug(data);
if (data.botId.indexOf(botId) === false) {
return Repo.UserBotModel.addUser(user.id, botId).then(function () {
data.botId.push(botId);
return data;
});
} else
return data;
});
});
};
self.getDisplayName = function (user) {
if (user.real_name)
return user.real_name;
if (user.last_name)
return user.firstname + ' ' + user.last_name;
return user.first_name;
};
/**
* check if user exist in our database/memory cache and return it,
* otherwise insert in the database and cache it in memory and the return it
* #param user
*/
function get(user) {
return new Promise(function () {
//check if user is loaded in our memory cache
if (self.users.hasOwnProperty(user.id))
return self.users[user.id];
else {
//get from database if exist
return Repo.UserModel.get(user.id).then(function (rows) {
if (rows && rows.length) {
//user exist cache it and resolve
var data = rows[0];
if (data.botId && data.botId.length)
data.botId = data.botId.split(',');
else
data.botId = [];
self.users[user.id] = data;
//------------------------------ code execution reaches here
return data;
}
else {
//user dose not exist lets insert it
return Repo.UserModel.insert(user).then(function (result) {
return get(user);
});
}
});
}
});
}
}
I call the start method witch calls the private get method the call reaches return data;(marked with comment) but then function dose not gets executed in the start method ???
So what am i doing wrong?
UPDATE : Sorry I forgot to mention that I am using bluebird and not the native promise if that makes a difference?

You cannot return from the Promise constructor - you have to call resolve (expected to happen asynchronously). You're not supposed to use the Promise constructor at all here. You can just omit it, and it should work.

The methods from your Repo.UserModel already return promises, so you do not have to create new ones using new Promise.
You can read the values inside those promises using then.
then also provides a way to transform promises. If you return a value in a function passed to then, then will return a new promise that wraps the value you returned. If this value is a promise, it will be awaited.
To convert a value to a promise, you can use Promise.resolve.
Knowing that, you can simplify get like so:
function get(user) {
if (...) {
return Promise.resolve(...)
} else {
return Repo.UserModel.get(...).then(function(rows) {
...
return ...
})
}
}
This version of getwill always return a promise that you can use like so:
get(...).then(function(resultOfGet) {
// process resultOfGet
})

how to break promise chain

I a promise in such fashion,
function getMode(){
var deferred = Promise.defer();
checkIf('A')
.then(function(bool){
if(bool){
deferred.resolve('A');
}else{
return checkIf('B');
}
}).then(function(bool){
if(bool){
deferred.resolve('B');
}else{
return checkIf('C');
}
}).then(function(bool){
if(bool){
deferred.resolve('C');
}else{
deferred.reject();
}
});
return deferred.promise;
}
checkIf returns a promise, and yes checkIf cannot be modified.
How do I break out of the chain at the first match? (any way other than explicitly throwing error?)

Any way other than explicitly throwing error?
You may need to throw something, but it does not have to be an error.
Most promise implementations have method catch accepting the first argument as error type (but not all, and not ES6 promise), it would be helpful under this situation:
function BreakSignal() { }
getPromise()
.then(function () {
throw new BreakSignal();
})
.then(function () {
// Something to skip.
})
.catch(BreakSignal, function () { })
.then(function () {
// Continue with other works.
});
I add the ability to break in the recent implementation of my own promise library. And if you were using ThenFail (as you would probably not), you can write something like this:
getPromise()
.then(function () {
Promise.break;
})
.then(function () {
// Something to skip.
})
.enclose()
.then(function () {
// Continue with other works.
});

You can use
return { then: function() {} };
.then(function(bool){
if(bool){
deferred.resolve('A');
return { then: function() {} }; // end/break the chain
}else{
return checkIf('B');
}
})
The return statement returns a "then-able", only that the then method does nothing.
When returned from a function in then(), the then() will try to get the result from the thenable.
The then-able's "then" takes a callback but that will never be called in this case. So the "then()" returns, and the callback for the rest of the chain does not happen.

I think you don't want a chain here. In a synchronous fashion, you'd have written
function getMode(){
if (checkIf('A')) {
return 'A';
} else {
if (checkIf('B')) {
return 'B';
} else {
if (checkIf('C')) {
return 'C';
} else {
throw new Error();
}
}
}
}
and this is how it should be translated to promises:
function getMode(){
checkIf('A').then(function(bool) {
if (bool)
return 'A';
return checkIf('B').then(function(bool) {
if (bool)
return 'B';
return checkIf('C').then(function(bool) {
if (bool)
return 'C';
throw new Error();
});
});
});
}
There is no if else-flattening in promises.

I would just use coroutines/spawns, this leads to much simpler code:
function* getMode(){
if(yield checkIf('A'))
return 'A';
if(yield checkIf('B'))
return 'B';
if(yield checkIf('C'))
return 'C';
throw undefined; // don't actually throw or reject with non `Error`s in production
}
If you don't have generators then there's always traceur or 6to5.

You could create a firstSucceeding function that would either return the value of the first succeeded operation or throw a NonSucceedingError.
I've used ES6 promises, but you can adapt the algorithm to support the promise interface of your choice.
function checkIf(val) {
console.log('checkIf called with', val);
return new Promise(function (resolve, reject) {
setTimeout(resolve.bind(null, [val, val === 'B']), 0);
});
}
var firstSucceeding = (function () {
return function (alternatives, succeeded) {
var failedPromise = Promise.reject(NoneSucceededError());
return (alternatives || []).reduce(function (promise, alternative) {
return promise.then(function (result) {
if (succeeded(result)) return result;
else return alternative();
}, alternative);
}, failedPromise).then(function (result) {
if (!succeeded(result)) throw NoneSucceededError();
return result;
});
}
function NoneSucceededError() {
var error = new Error('None succeeded');
error.name = 'NoneSucceededError';
return error;
}
})();
function getMode() {
return firstSucceeding([
checkIf.bind(null, 'A'),
checkIf.bind(null, 'B'),
checkIf.bind(null, 'C')
], function (result) {
return result[1] === true;
});
}
getMode().then(function (result) {
console.log('res', result);
}, function (err) { console.log('err', err); });

i like a lot of the answers posted so far that mitigate what the q readme calls the "pyramid of doom". for the sake of discussion, i'll add the pattern that i plunked out before searching around to see what other people are doing. i wrote a function like
var null_wrap = function (fn) {
return function () {
var i;
for (i = 0; i < arguments.length; i += 1) {
if (arguments[i] === null) {
return null;
}
}
return fn.apply(null, arguments);
};
};
and i did something totally analogous to #vilicvane's answer, except rather than throw new BreakSignal(), i'd written return null, and wrapped all subsequent .then callbacks in null_wrap like
then(null_wrap(function (res) { /* do things */ }))
i think this is a good answer b/c it avoids lots of indentation and b/c the OP specifically asked for a solution that doesn't throw. that said, i may go back and use something more like what #vilicvane did b/c some library's promises might return null to indicate something other than "break the chain", and that could be confusing.
this is more a call for more comments/answers than a "this is definitely the way to do it" answer.

Probably coming late the party here, but I recently posted an answer using generators and the co library that would answer this question (see solution 2):
https://stackoverflow.com/a/43166487/1337392
The code would be something like:
const requestHandler = function*() {
const survey = yield Survey.findOne({
_id: "bananasId"
});
if (survey !== null) {
console.log("use HTTP PUT instead!");
return;
}
try {
//saving empty object for demonstration purposes
yield(new Survey({}).save());
console.log("Saved Successfully !");
return;
}
catch (error) {
console.log(`Failed to save with error: ${error}`);
return;
}
};
co(requestHandler)
.then(() => {
console.log("finished!");
})
.catch(console.log);
You would pretty much write synchronous code that would be in reality asynchronous !
Hope it helps!

Try to use libs like thisone:
https://www.npmjs.com/package/promise-chain-break
db.getData()
.then(pb((data) => {
if (!data.someCheck()) {
tellSomeone();
// All other '.then' calls will be skiped
return pb.BREAK;
}
}))
.then(pb(() => {
}))
.then(pb(() => {
}))
.catch((error) => {
console.error(error);
});